Conventionally, in power distribution systems of the type employed in aircraft, each load circuit incorporates both a relay for switching current and a thermal circuit breaker to protect the circuit wiring from overloads. The relay and circuit breaker for many circuits are located in the cockpit for flight crew operation requiring heavy gage wire to run from the generator to the cockpit and then to the load resulting in a substantial weight penalty.
In U.S. Pat. No. 4,866,559 a solid state protective circuit is disclosed which is capable of remotely switching power to a load in which an electrothermal sensor is positioned in heat transfer relationship with a resistive element in series with the power line to the load to monitor the current to the load and provide a signal to control logic indicative of the sensed current determined from the amount of heating caused by the current flow through or in the line coupled to the electrothermal sensor, the amount of heat being determined by the electrothermal sensor.
Another solid state power controller is shown and described in copending application Ser. No. 985,406, filed on Dec. 4, 1992 assigned to the assignee of the present invention. The controller in the referenced application uses a shunt resistor as a current sensing mechanism. The shunt resistor is adapted to conduct the full current load from the power generator to the load and the controller measures the voltage drop across the resistor apparatus and processes this measurement to limit current to the load to a safe level. The controller limits current in accordance with a selected curve of time versus percent overload current. An example of a controller of this type comprises a hybrid assembly having a substrate on which are mounted selected ASICS, FETS, resistors, capacitors and a back-up fuse as well as the shunt resistor in a package in the order of two inches in length, one and a third inches in width and a third of an inch in height.
One of the problems in providing a shunt resistor for use with the controller is the small amount of space available for the resistor and the need to conform with standard hybrid assembly techniques. The shunt resistor must be able to dissipate the power that the controller is designed to handle and still be able to be of a size and type to be mounted within the controller package. Conventional discrete resistors which have the ability to handle the required power are too large to fit within the package. Conventional thick film resistors are not suitable because of their limitations in power dissipation. Other devices which are unsuitable include plastic encapsulated wire welded to contacts. While these devices can handle the normal steady state power loads they are not able to handle the required overload and as a result overheat on such overloads and crack or even break out of their encapsulants due to wire expansion.